The forward-most seacock on the starboard side on Rainshadow is the toilet inlet seacock. This is accessed through the floor board just inside the v-berth cabin. It is very close to the centerline of the hull, adjacent to the wall that separates the toilet from the aisle into the v-berth area. Ours is a 3/4″ Blakes tapered cone seacock. Is this the one you mean?
Further forward, far above the waterline so without a seacock, there is a drain for the forward deck anchor locker.
If neither of those are in the location your describe, then I would guess this must be an after-build installation by a previous owner. Perhaps for a former watermaker, where the PO wanted to collect water before the toilet outlet?
Marilyn